FLEXNET – FNER Expandable (Modular) Routing Switches – Installation & Operation Manual
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CHAPTER 6: RS-485
RS-485
6. RS-485
6.1 Why an RS-485 Section?
In the past, there have been questions as to how to handle certain RS-485 communications setups with FNER systems. This
section was written to eliminate confusion about the FNER RS-485 communications, specifically termination. This section
discusses the major points of RS-485 communications, including specific details such as signaling, cabling, pinout, and
termination settings.
6.2 RS-485 Basics
In order to discuss RS-485 lets review the basics of RS-232 for comparison. RS-232 in simplest terms is a three-wire
interface: one transmit, one receive, and one common ground. This is called an unbalanced signal.
It’s a simple protocol but because signaling is in reference to a common ground, the speed and cable lengths are severely
limited. RS-232 is typically only a few feet and usually limited to less than 20,000 bits per second. Rather slow for today’s
standards.
6.2.1 Balanced vs. Unbalanced
RS-485 standard uses balanced differential signals. In balanced signaling there are two lines. One signal carries the true
signal and the other carries the same signal except 180 degrees out of phase. The receiver then reads the difference
between the two signals. A basic diagram is shown in Figure 19. A ground wire does exist but it has nothing to do with how
the receiver determines the logic level of the signal.
Figure 19: Balanced Line Interface
6.2.2 Voltages & Signaling
A pair of wires is used to carry each signal. The data is encoded and decoded as a differential voltage between the two lines.
A typical truth table is shown below.
VA-VB < -0.2v = Logic 0
VA-VB > +0.2v= Logic 1
As a differential voltage, in principle the interface is unaffected by differences in ground voltage between sender and receiver.
6.2.3 Noise Immunity
Furthermore, if lines A and B are close together, they will be affected almost identically by external noise. If the lines are also
twisted together, then neither line is permanently closer to a noise source than the other. Hence, the well known “twisted pair”
is extremely effective in eliminating noise from the signal. Due to the nature of the differential receiver, any equivalent noise
on both wires is cancelled out.
6.2.4 Number of Devices
RS-485 standard allows 32 devices on one bus where RS-232 is typically only a one to one connection. The MPC 128 can
be on an RS-485 bus with up to and including 31 other devices.
Line A
Line B
Transmitter
Receiver
Balanced Line Interface
